The evidence base across skincare is uneven. Some categories rest on well-conducted trials with clear outcomes, and a great deal rests on small studies funded by manufacturers, or on plausible mechanisms that have not been tested in people.
Sun protection sits at the strong end, and it is the thing people apply least consistently.
General information only. Anything about your own skin belongs with a doctor.
Why the evidence is stronger here
Because the outcomes are serious and therefore properly studied.
Ultraviolet exposure has a well-established causal relationship with skin cancers, and that literature is extensive, long-running and independent of the cosmetics industry.
Photoageing — the changes in skin appearance attributed to sun exposure — has also been studied, including in controlled trials examining whether daily sunscreen use affects visible ageing over years.
Which means the claims here are supported by the kind of evidence that most cosmetic claims are not.
The application problem
Where practice diverges from the evidence.
The protection factor on a product is determined under standardised testing using a specified quantity per area of skin.
Studies of how much people actually apply consistently find substantially less than that — frequently a quarter to a half.
Protection does not scale linearly with quantity, so applying half the tested amount does not give half the stated factor; it gives considerably less.
Which means the number on the bottle describes a laboratory condition that most users do not reproduce, and the practical protection is lower than the label suggests.
The areas people miss
Documented in studies using imaging that reveals coverage.
The eyelids and the area around the eyes, consistently, which is also where a large share of certain skin cancers occur.
The ears, the hairline, the back of the neck, the tops of the feet, and the lips.
These are missed reliably rather than occasionally, and knowing the list is most of the remedy.
The reapplication question
Where the practical guidance is clear and widely ignored.
Protection degrades through the day — rubbing, sweating, water, and to some extent the breakdown of filters.
Guidance generally recommends reapplication every couple of hours during sustained exposure, and after swimming or heavy sweating.
For anybody indoors most of the day, the practical question is different and less demanding, and applying in the morning without reapplication is a reasonable position for that pattern.
For anybody outdoors, reapplication is where most of the failure happens.
The filter debate
Worth summarising since it generates a lot of noise.
Filters divide broadly into mineral and organic types, which work by different mechanisms.
Both are assessed by regulators and both are permitted, with the specific permitted list differing between jurisdictions, which is why products differ between markets.
Questions have been raised about systemic absorption of certain organic filters, and regulators have requested further data on some. That is a request for information rather than a finding of harm, and it has been widely reported as though it were the latter.
The considered position from dermatology bodies has generally been that the established benefit is substantial and the theoretical concerns are unresolved, which is a reasonable summary of where the evidence sits.
What actually makes people use it
The practical point that matters more than the chemistry.
The best product is the one applied daily in sufficient quantity, and that is determined by texture, finish and whether it is unpleasant to wear.
Formulations have improved enormously in this respect, and the products that were genuinely disagreeable to use a decade ago are not representative of what is available now.
Anybody who abandoned it on the basis of an old experience is working from outdated information.
The things that are not sunscreen
Worth noting because they matter more than product choice.
Shade, timing, and clothing, all of which are more effective than any topical product and none of which requires reapplication.
Fabric provides protection that varies with weave and colour, and garments rated for it are available.
And a hat with a brim addresses the ears, neck and eye area, which are the places most reliably missed.
Vitamin D, briefly
The objection that always arrives and deserves an honest answer.
Sun exposure produces vitamin D, and comprehensive sun avoidance can affect status, particularly at higher latitudes and for people with darker skin.
The relationship between typical sunscreen use and measured vitamin D status has been studied, and real-world use does not appear to produce deficiency in most populations, largely because application is incomplete.
Where deficiency is a concern, supplementation is the remedy that health bodies generally recommend, since it addresses the issue without the associated risk.
This is exactly the sort of question to raise with a doctor rather than to resolve from an article.
The numbers on the bottle
Since two different ratings appear and they measure different things.
One describes protection against the shorter wavelengths primarily responsible for burning.
The other, indicated variously depending on the market, describes protection against longer wavelengths associated with ageing and contributing to other damage.
A high figure for the first with poor protection against the second is possible, which is why broad spectrum labelling matters and why the systems for indicating it differ confusingly between regions.